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The high-pressure direct-injection (HPDI) of natural gas in a compression ignition engine has
the potential to reduce demand for petroleum derived fuels and significantly reduce the level
of pollutants and greenhouse gases emitted from heavy duty transport vehicles. A new HPDI
injector was tested where diesel is injected into a gas/diesel reservoir in the injector and the
diesel and gas are then co-injected into the combustion chamber. In order to identify
interactions between the diesel and gas in the reservoir, two different injector geometries
were tested: prototypes A and B. Prototype B had reduced reservoir volume to increase gas
velocity inside the injector.
A majority of the tests were conducted in a single-cylinder test engine derived from a
Cummins ISX diesel engine. As prototype A was being modified to create Prototype B this
test engine was moved to a larger test cell. After updating the electrical, mechanical, and
safety systems, the test engine in the new test cell was found to run repeatably; however,
emissions comparisons between both test cells was not possible due to different analyzers
being used.
Single gas and double gas injections were conducted for both injector prototypes. The single
gas injection tests found that increasing the diesel injection mass reduced the mass of gas
injected. Increased diesel injection mass also shortened ignition delay, reduced unburned and
partially burned fuel and increased NOx emissions. Holding the diesel injection mass
constant and reducing the gas injection mass had the same effect as increasing diesel on
ignition delay and gaseous emissions. If the diesel injection mass was kept constant and a
second gas injection was added, the heat release due to the first injection decreased and the
start of combustion was retarded. This appears to have occurred because some of the diesel
was carried into the cylinder by the second injection and less diesel was available in the first
injection to promote ignition.
Double gas injection tests were conducted where the load, speed, and combustion timing
were controlled in order to determine how injector operation affects parameters such as
knock intensity, and gaseous emissions. At lower diesel injection masses, retarded
combustion timing led to shorter ignition delays and less intense knock and lower unburned
fuel emissions at lower loads. Longer relative times between the diesel and gas injections
had a similar effect as lower diesel injection mass, especially at advanced combustion timing.
For these tests Prototype B exhibited shorter ignition delays but higher knock intensities than
Prototype A.

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